A fuel cell is an electrochemical cell which consumes fuel and an oxidant on a continuous basis to generate electrical energy. The fuel is consumed at an anode and the oxidant at a cathode. The anode and cathode are placed in electrochemical communication by an electrolyte. One typical fuel cell employs a phosphoric acid electrolyte. The phosphoric acid fuel cell uses air to provide oxygen as an oxidant to the cathode and uses a hydrogen rich stream to provide hydrogen as a fuel to the anode. After passing through the cell, the depleted air and fuel streams are vented from the system on a continuous basis.
A typical fuel cell power plant comprises one or more stacks of fuel cells, the cells within each stack being connected electrically in series to raise the voltage potential of the stack. A stack may be connected in parallel with other stacks to increase the current generating capability of the power plant. Depending upon the size of the power plant, a stack of fuel cells may comprise a half dozen cells or less, or as many as several hundred cells. Air and fuel are usually fed to the cells by one or more manifolds per stack.
In each of the fuel cells, waste heat is a by-product of the steam reforming process for conversion of natural gas to a hydrogen rich steam, electrochemical reactions, and the heat generation associated with current transport within the cell components. Accordingly, a cooling system must be provided for removing the waste heat from a stack of fuel cells so as to maintain the temperature of the cells at a uniform level which is consistent with the properties of the material used in the cells and the operating characteristics of the cells.
In the stack, where the chemical reactions take place, water is used to cool the stack and generate steam to be used in the fuel processing system, where chemical reactions occur to generate hydrogen. The waste heat, which is at around 350° F. and includes water, exit air and depleted fuel, is directed to a waste heat recovery loop to provide the customer with low grade heat (i.e. up to 140° F.). The heat recovery loop often includes a condenser coupled with a glycol loop and a heat exchanger that couples the water system with the glycol loop. The customer can also get high grade heat (i.e. up to 335° F.) via the water which receives heat from the stack cooling loop. The temperatures for high grade heat and low grade heat will vary depending upon the fuel cell system (e.g., solid oxide or molten carbonate). The high temperature allows the fuel cell power plant to be utilized in a cogeneration power system.
A fuel cell cogeneration power system, or fuel cell combined heat and power (CHP) system, generates electrical and thermal energy in a single, integrated system. Typical components of a fuel cell CHP system include a fuel cell power plant as the electrical generator, an electrical interconnection coupled to the fuel cell power plant, and a heat recovery system coupled to the waste heat generated by the fuel cell power plant. The heat recovery system is used to meet the direct thermal demands of a facility or process, such as steam or hot water. Alternately, the waste heat may be used as a heat source for an absorption/chiller system to meet indirect facility thermal load demands, such as chilled water for facility cooling and heating.
Typical fuel cell power plants operate either in a grid-connect mode or grid-independent mode. In grid-connect mode, the operator or site controller provides a constant electrical power set point for the fuel cell power plant. Any facility electrical demand in excess of the set point is supplied by the grid. In grid-independent mode, the fuel cell power plant is the sole provider of electrical power for the site, and the electrical output of the fuel cell power plant varies according to the electrical demand of the site.
One drawback to both types of systems is that the fuel cell power plant is configured to generate electrical power as a first priority, and the thermal power output—whatever it may be—is delivered to the heat recovery system as a secondary consideration. In either mode, the waste heat generated by the fuel cell power plant is an uncontrolled parameter and will vary based upon system variables such as power level, operating time, and electrical efficiency, to name a few. This operating limitation may frustrate site operators who place a premium on satisfying thermal demands, such as when the cost of such thermal demands is proportionately more expensive than satisfying the electrical demands.